Signal transmission between a mobile device and a base station is enhanced through mobile transmission diversity (MTD), sometimes referred to as beam forming. One form of MTD employs multiple antennas in the mobile unit, each antenna transmitting identical data. The phase difference between the signals from the two or more antennas is controlled so that constructive interference of the signals received at a base station provides power received at the base station greater than the arithmetic sum of the received power that would be radiated from each of the antennas in the absence of the other(s). By varying the phase difference between the signals from the antennas, the peak radiated power may be steered to focus upon the antenna of the base station. Typically two antennas are used for MTD.
Since the phase of a signal reaching a base antenna is affected by changes of path length such as due to reflections, closed loop MTD has been developed where the phase difference at the multiple mobile unit antennas is adjusted in response to feedback from the base station. In particular a quality factor such as bit error rate, or a Power Control Bit (PCB) or Transmitter Power Control (TPC) parameter determined at the base station may be communicated back to the mobile device, or the base station may return a signal indicating whether the received power should be lowered or raised. This communication from the base signals the need to adjust the phase difference between the antenna outputs.
Since the mobile device is located close to a user, the amount of radiation impinging on the user is also monitored. The Specific Absorption Rate (SAR) is a measure of the amount of power absorbed by biologic tissue and a goal is to keep the SAR below a predetermined value. Absorption levels may be typically defined and measured by placing a liquid-filled phantom head, hand, or other emulated body part close to the edge(s) of a mobile device while transmitting. Measurements of a rise in the liquid's temperature provide an indication of the radiation exposure overall or at particular points
To meet SAR requirements the conventional approach may be to set the antenna radiation limits based on peak radiation points rather than peak radiated averages (i.e. total radiated power (TRP)). This conventional approach may cause vendors to limit the maximum power and or the maximum data rate.